Organic electroluminescence (“EL”) display devices of a type comprising a matrix of a large number of self-luminous organic EL elements hold great promise as the next generation of image display devices since they require no back-lighting nor do they restrict viewing angles.
The organic EL elements are light-emitting elements of a current-driven type, of which brightness can be controlled by an amount of electric current flowed through them. There are simple matrix type and active matrix type as the methods of driving the organic EL elements. The former has a drawback that it is difficult to produce a large-scale and high-definition display although it only needs simple pixel circuits. It is for this reason that the efforts are being made actively in recent years for development of organic EL display devices of the active matrix type, which is composed of a matrix of pixel circuits having organic EL elements, each provided with a driver transistor for driving the current-driven type light-emitting element.
The driver transistor and the peripheral circuit are formed generally of thin film transistors. There are thin film transistors of a type made of polysilicon and another type made of amorphous silicon. The amorphous silicon thin-film transistors are suitable for large-scale organic EL display devices since they feature a high uniformity in mobility, easy to fit for upsizing, and inexpensive, although they have some weaknesses such as poor mobility and large changes in the threshold voltage with time. There have been some studies conducted for measures to overcome the weakness, or the changes in the threshold voltage with time, of the amorphous silicon thin-film transistors by improvements of the pixel circuits. Patent document 1, for instance, discloses an organic EL display device having pixel circuits capable of displaying stable images by keeping an amount of the currents supplied to the light-emitting elements free from influence of the threshold voltage of thin film transistors even when the threshold voltage changes.
In addition, patent document 2 discloses a compensation circuit as a more advanced compensating function for compensating degradation of organic EL elements in order to achieve further extension of useful life of display devices.
However, the pixel circuit disclosed in the patent document 1 is comprised of P-channel transistors. On the other hand, there are only N-channel transistors that are now available as amorphous-silicon thin-film transistors for practical use in large-scale image display devices, and it is therefore necessary to compose the image circuits only with N-channel transistors. In addition, it is also preferable that the pixel circuits have a structure allowing connections of anodes of organic EL elements to sources of driver transistors and cathodes of the organic EL elements to a common electrode so as to ease the manufacturing of organic EL display devices. Furthermore, there exists the need for pixel compensation circuits operable in a source-grounded structure in order to reduce unevenness in luminous brightness, which is liable to occur due to voltage drops resulting from electrical resistances of power lines and currents flowing therethrough when the organic EL elements are lit.
The circuit for compensating degradation of the organic EL elements as disclosed in the patent document 2 has a source-grounded structure designed to use P-channel transistors, and it is therefore impossible to achieve the circuit by using amorphous silicon transistors, which offer no choice but only of the N-channel type.
[Patent Document 1] Japanese Translation of PCT Publication, No. 2002-514320
[Patent Document 2] Japanese Patent Unexamined Publication, No. 2006-309104